Tiny homes, tiny wood stoves: photos, ideas and designs

With the advent of the tiny home movement, there is a rise in interest in tiny stoves to heat them.  Tiny stoves have always been around, mainly driven by the sailboat industry, but also for yurts and small homes.  Small stoves are often thought of as stoves with a firebox of less than one cubic foot.  But some are much smaller than that and may put out no more than 10,000 BTUs.  There will also likely be a growing market for very small pellet stoves, like the Thelin Gnome, as living spaces get smaller and tighter. 
The paradox of heating a small space is that it may not be hard to heat up, but it also gets cold quickly after the stove goes out.  The fireboxes are so small that they go out quickly.  Often, tiny stove need to be reloaded every 20 – 60 minutes, depending on the size of the fuel and whether the stove is just getting going or has a decent coal bed.
A few of these stoves are EPA certified, including the Kimberly and the Gnome pellet stove.  If they are designed for boats, vans, trailers or for camping or other non-residential spaces, they should fall outside the EPA’s regulations, which only pertain to residential heating.  However, even if they do not require to pass EPA emissions testing to go on sale, they may not be allowed to be installed in a tiny home.  See our other photo essays on wood stoves styles around the world, wood fired hot tubs and firewood gathering around the world.

To minimize space, tiny stoves can be mounted on the wall. Using wood stoves in boats, vans and tiny homes can pose a great risk of carbon monoxide build up than in larger spaces. Be sure to install a CO detector, store your ashes outside and ensure the draft doesn’t reverse back down the chimney.
Tiny stoves are often installed on counters or shelves so that operating and cooking on them is easier.

Yurts are traditionally heated with larger, inefficient stoves, not small, sleek ones like this.  


The Gnome pellet stove is the smallest pellet stove on the market and claims to run for more than 24 hours on one hopper load of pellets at low heat.

The Viking 30 cookstove is part of a retro line of wood stoves from the UK.

A small stove in a classic Airstream trailer.

Heated Up!

Maryland program helps residents install cleanest stoves on the market

Chris Clark manages the clean wood and
pellet stove program for the Maryland
Energy Administration.
A Maryland renewable energy rebate program established in 2012 has provided assistance to nearly 3,000 to purchase and install cleaner, modern wood and pellet stoves.  The program was designed to help rural families, who were least likely to benefit from solar and other renewable energy programs.
The state has invested $ 1.88 million dollars, or an average of $ 664 per home.  The average purchase and installation costs of a stove is often around $ 3,800, so the state would be providing less than 20% of the price tag for a significant reduction in a home’s annual fossil fuel footprint.
The Maryland Energy Administration who manages a suite of renewable energy incentives runs the program.  With this program, the MEA cannot exactly track the amount of fossil fuel that the program has reduced, which hinders the ability of regulators to track data as they can with the solar and other rebate programs.
Of the 2,845 stoves purchased through the program, 2,425 or 85% were pellet stoves.  To be eligible, pellet stoves have to emit 2 grams an hour or less, and wood stoves 3 grams an hour or less, well below the federal EPA limit of 4.5 grams an hour.  Pellet stoves are eligible for a $ 700 grant and wood stoves $ 500.

Rebate data provided by MEA
“We are pleased that this program steers so many people towards pellet stoves and ensures that the installation is done professionally,” said John Ackerly, President of the Alliance for Green Heat, a Maryland based group that worked with MEA to establish the program. 
Only residents who do not have access to natural gas are eligible for the grant, which as a result helps families in rural areas who rely on more expensive fossil heating fuels.  New York also adopted this innovative approach, and only provides incentives to homes that are not on the natural gas grid.  A 2013 analysis of the program by the Alliance for Green Heat found that it was helping less affluent families to reduce fossil fuel use.
The program does not require residents to turn in an old, uncertified wood stove to participate in the program.  However, some retailers report that nearly half of their customers who use this program turn in an old stove that is recycled. 
During 2015, the program experienced its highest participation rates, providing grants for 1,036 stoves, 904 of which were pellet stoves. 

“A $ 3,000 pellet stove installation can reduce fossil fuel usage by as much as a $ 15,000 array of solar panels,” said Ackerly.  “And increasingly, we see families who have solar panels also installing pellet stoves so that they can use renewable energy for both their electric and heating needs,” he added.

Heated Up!

HHT and Ravelli concur to launch performance for 5 stoves

In reaction to a Independence of Information Act request filed by the Alliance for Inexperienced Warmth, Fireside &amp Residence Technologies (HHT) and Ravelli unveiled real effectiveness figures for many of their common pellet stove models.&nbsp Efficiencies ranged from fifty six% to 76%.
Wooden and pellet stoves certified after May 2015 must take a look at for and disclose their performance to consumers, as necessary by new EPA restrictions. Prior to these restrictions, stove producers claimed that any accessible performance information was private and they usually did not give it to consumers.&nbsp Nevertheless, several organizations provided the performance information to the EPA as element of the certification process.&nbsp
The Alliance for Environmentally friendly Warmth, based on its legal counsel, argued that the EPA could not keep effectiveness knowledge it gathered prior to May possibly 2015 private since it was based mostly on emission knowledge, and emissions data can’t be private.&nbsp In July 2015 the EPA declined to launch the efficiencies to the Alliance, in keeping with its longstanding apply that they would hold all data private if it was submitted as Private Business Details (CBI). &nbsp
At first, the Alliance experienced questioned for the weighted typical efficiency as nicely as the effectiveness from each and every of the 4 burn charges that labs use to take a look at stoves.&nbsp The Alliance agreed to drop the ask for for the individual burn price efficiencies and only request the average.&nbsp Primarily based on that adjust, HHT and Ravelli agreed to allow the EPA to launch their effectiveness figures to the Alliance with some conditions.
The EPA provided a closing reaction with a data sheet (pdf) hooked up.&nbsp The performance figures (HHV based on B415.1) from that knowledge sheet are:
Organization&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp Model&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp Effectiveness
HHT&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp Accentra 52i insert&nbsp &nbsp seventy six%
HHT &nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp Mt Vernon E2&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp seventy five%
HHT&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp Mt Vernon AE&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbspseventy one%
Ravelli&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp RV100&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp70%
Ravelli&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp RV80&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbspfifty six%
The Alliance asked for effectiveness data for these certain models, together with numerous other folks, due to the fact we have been in the process of independently tests a assortment of pellet stoves.&nbsp The Alliance was also attempting to encourage the EPA to change their coverage and disclose all effectiveness knowledge, even if that performance information had been marked as Private Organization Details.&nbsp The Alliance argued &nbsp&nbsp
The Ravelli RV80 was tested at
fifty six% efficiency by a EPA
accredited test lab.

that performance was of these kinds of standard significance to shoppers and that maintaining it confidential served no valuable goal. That try was unsuccessful. &nbspThe Alliance is not appealing.

The 5 freshly unveiled real effectiveness figures are not incorporated on the EPA’s record of qualified stoves, the place about 20% of the stoves have real efficiencies outlined. &nbspExcept for a single of the Ravelli’s, these five stoves all have about average or above common efficiencies.&nbsp&nbsp The Ravelli RV80 is only fifty six% successful.&nbsp Nearly all stoves assert to be seventy five% successful to be qualified for the $ three hundred federal tax credit, and EPA listing or disclosure that reveals the stove is not even 60% successful does not appear to deter companies from continuing to claim eligibility for the tax credit rating.&nbsp &nbsp

Heated Up!

Downdraft Wooden Stoves: Consumers Beware

The simplicity of the non-catalytic stove helps make it the most well-known sort of stove in North The united states by much.&nbsp With most non-cats, the buyer only requirements to fret about a one lever to regulate the quantity of air in the firebox.&nbsp But some shoppers unwittingly buy a extremely unique sort of non-cat, a &nbspdowndraft stove, and often regret it.

These days there are only a handful of downdraft stoves on the marketplace, Most have been pushed apart by non-cat stoves with air tubes that reburn smoke as it exits the firebox.&nbsp The downdraft stove works with a next lever that engages a by-move damper which forces the smoke down into a chamber the place heat can reburn it ahead of it goes again up the chimney.&nbsp
Nearly all stoves on the industry today do not consider to get smoke to go down, but rather try out to reburn it on its journey upward with secondary air launched by way of holes in tubes or in higher wall of stove – or with a catalyst. Hybrids use the two tubes to inject for secondary air and a catalyst. &nbsp(Click below for a lot more on cat, non-cat and hybrid stoves.)
Downdraft stoves tend to be finicky, require to be really sizzling to function appropriately and are far more inclined to leaking smoke into the property.&nbsp Most buyers must not take into account them until they know what they are obtaining into and are ready to be a lot a lot more engaged with their stove.&nbsp The same can be said about catalytic stoves.&nbsp Customers should comprehend that they need to know how to use the next lever that engages the by-move damper and from time to time check the catalyst, and exchange it when it requirements replacing.

Stove modify out applications, that switch out outdated stoves with new, EPA certified types could want to exclude downdraft stoves from eligible replacements as they could be the most difficult class of stove to operate regularly without having seen smoke.

The downdraft stove turned common in the 1980s when Vermont Castings employed it to extract much more heat from a stove and to support go the emission testing required by the state of Oregon, and later by the EPA. &nbspThe by-go damper aided sluggish down the hearth, create turbulence, and extract a lot more warmth from the very hot gases ahead of they went up the stack. The Resolute Acclaim, a single of the most well-known wooden stoves of the final century, was a downdraft stove.&nbsp It clocked in at 3.4 grams for each hour and ongoing selling via the 2000s, but has given that been taken out of creation.
Today, there are a handful of downdraft stoves on the industry, including the Lopi Leyden and the Avalon Arbor (which are evidently the same stove).&nbsp The&nbspHarman TL300 is also a downdrafter and Vermont Castings still has one particular or two.
The authentic Frankin stove
was a downdraft with no
front door and typically
created for a smoky house.

Downdraft stoves have existed for centuries in numerous patterns. The authentic stove invented by Ben Franklin also experienced a downdraft included into the layout, and it is a single of the causes that the stove was so finicky and disfavored by the greater part of buyers. Franklin himself admitted that the stove was tough to function and took lots of interest and was not anything that “could be left to the servants.” &nbspLater iterations of the Franklin stove included a doorway and most received rid of the downdraft.

Customer feedback
Hearth.com is a leading chat place for wood stove specialists and rookies alike. It consists of a good deal of very good suggestions on downdraft stoves:
– “The creosote smell when closing the bypass is not an uncommon situation with the downdrafting stoves. I’ve seen Harman stove owners complain with it as nicely. I will often get it. Occasionally not. It is not noxious, just a faint odor that I can do with out. CO keep track of pays it no head.”
– “I want a team of downdrafters could get their heads collectively on this a single and figure out what is likely on. It really is a frequent problem to different stoves. Possibly that box complete of smoke is permitting some escape at the joints.”
– “I’ve smelled it when the draft was robust, but typically it comes when the flue has begun to awesome down a little. Loading lid was a key suspect, but I regasketed it and sat a solid iron pot on there.”
– “Downdraft stoves are notoriously fussy and demanding of a robust draft.”
– “The Leyden has gone through a handful of modifications of the several years. There are some ports that require complete cleaning “Regular monthly” as properly and they are not effortless to get to. Make certain these ports are cleaned frequently. Your guide shows how to do this properly.”
– “These stoves have to be critically very hot to perform.”
– “I occasionally get a bit of smoke when loading. Mainly due to the fact I am way too lazy to do it proper. Once more, operate the stove as developed and if it will not perform, get a appear at the flue, check out for ash buildup, etc.. … your flue style is gonna be a tough one.”
– “I don’t want to melt your stove down, but my hunch is that you might be not acquiring it sizzling enough on a three-four” coal mattress.”
– “Is the wooden really properly seasoned? That is very important with this stove as is a good draft.”
– “I would try working your stove leading temperature a small larger (650-seven-hundred) prior to you go to secondary melt away, generally you get some fall in stovetop temperature when you near the damper.”
– “The Lopi Leyden is a extremely problematic stove. You don’t want it, imagine me!”
– “I also propose to consider off the Vermont Castings Resolute and Lopi Leyden. Equally use a various system to obtain a thoroughly clean burn up than the other non-catalytic stoves. The two stoves typically have a steeper understanding curve, can be vulnerable to expensive repairs and are greater suited for entire-time burners which does not look to be your intention.”
– “As other individuals have talked about, the Oakwood (and other downdrafts like Lopi Leyden and specified Vermont Castings types) can be a little bit fussy about dry wood and good draft, requires a little bit a lot more babysitting and for a longer time studying curve, and is not well suited to slower/reduced burns in months like November and April.”
– “You want a stove with tubes in the leading or a catalyst. Avoid stoves that attract smoke down into the back of the stove. For instance: Vermont Castings, some Harman’s and Lopi Leyden.”

Heated Up!

Wooden and pellet stoves with true efficiency quantities

The following seventy one wood and pellet stoves have been tested by accredited laboratories for efficiency. Buyers should consider acquiring 1 of these stoves if they want a stove that has a trustworthy effectiveness value. &nbspHowever, with wood stoves, the shown efficiency signifies what shoppers can get if they use dry wooden and give the stove ample air. &nbspWith pellet stoves, the performance described by the lab is a great approximation of the effectiveness the customer can expect. &nbsp

Consumers need to beware that most producer websites put up unreliable and exaggerated efficiencies, besides for a tiny team of businesses. &nbspSome businesses could not put up the efficiencies of their pellet stoves due to the fact they are poorly developed and have efficiencies below sixty five%. Simply click right here for an analysis of stove efficiencies.
How to go through this chart: Some of the Model Names have been shortened to in shape into the structure of this weblog. &nbspTo check out the formal supply of accredited stoves that would include the total product name(s), refer to the record of EPA licensed stoves. &nbspThe emission price is the quantity of grams of particulate issue that make up smoke, per hour. The genuine efficiency refers to a typical of the range of heat outputs that the stove is capable of, calculated using the CSA B415.one-ten, which makes use of the larger heating worth (HHV). Below type of stove, “Cat” refers to catalytic stoves, “Hybrid” refers to stoves that have the two catalytic and non-catalytic engineering, and “Non Cat” refers to non-catalytic stoves.

Manufacturer Title Product Identify Emission Fee G/Hr True Performance&nbsp Kind
Kuma Stove Inc. Sequoia 2.2 eighty four Cat&nbsp
Blaze King Industries KEJ 1107&nbsp 1.eight eighty two Cat&nbsp
Woodstock Soapstone Best Metal&nbsp 1. 82 Hybrid&nbsp
Blaze King Industries Princess PEJ &nbsp two.four 81 Cat&nbsp
Blaze King Industries Princess 35&nbsp 2.one eighty one Cat&nbsp
Ningbo Hongsheng Comfortbilt HP5 2.five 81 Pellet
Regency (FPI) Regency 3500 one.1 81 Cat&nbsp
Woodstock Soapstone Development Hybrid&nbsp one.three eighty one Hybrid&nbsp
Blaze King Industries Ashord Chinook&nbsp .eight 80 Cat&nbsp
Blaze King Industries Princess Insert &nbsp 2 eighty Cat&nbsp
Regency (FPI) LG Flushwood one.eight 80 Cat&nbsp
Travis Industries Insert Hybrid&nbsp .six eighty Hybrid&nbsp
Travis Industries Cape Cod&nbsp .five eighty Hybrid&nbsp
England’s Stove Functions 25-SSP01 .56 seventy nine Pellet
Regency (FPI) F5100&nbsp 1.5 seventy nine Cat&nbsp
Travis Industries Rockport .eight seventy eight Cat&nbsp
Seraph Industries Genesis 108 2.1 seventy eight Pellet&nbsp
Blaze King Industries Chinook Sirocco&nbsp one.three 77 Cat&nbsp
Jotul North The united states&nbsp F55 3.5 seventy six Non Cat
Travis Industries Modest Flush &nbsp&nbsp .nine 76 Hybrid&nbsp
Blaze King Industries Chinook Sirocco&nbsp one. 75 Cat&nbsp
Jotul North The us&nbsp F118 CB three.5 75 Non Cat
Pacific Power Hearth TN20 two.one seventy five Non Cat
Pacific Power Hearth Neo 1.six, Insert three.nine 75 Non Cat
Pacific Energy Fireplace&nbsp Neostone 1.6 three.four seventy five Non Cat&nbsp
Sherwood Industries Enviro 1700l 4.five seventy five Non Cat
Jotul North The usa&nbsp F600 four.one 74 Non Cat
Jotul North America&nbsp F45 2.3 74 Non Cat
Jotul North The united states&nbsp F500 three.two 74 Non Cat
Pacific Strength Hearth Neo two.5, insert&nbsp two.nine seventy four Non Cat&nbsp
Travis Industries Evergreen three.six seventy four Non Cat
Travis Industries 42CVT Wooden .seven 73 Cat&nbsp
American Strength Sys. Baby Country S one seventy three Pellet
Boru Stove Firm Carraig Mor&nbsp three.9 seventy three Non Cat&nbsp
Jotul North The usa&nbsp F118 Black Bear three 73 Non Cat
Jotul North The usa&nbsp Tamarack 4.four 73 Non Cat
Jotul North The us&nbsp F3CBII 3.8 73 Non Cat
Jotul North The us&nbsp F602 CB 3.four seventy three Non Cat
Jotul North The usa&nbsp F100 Nordic three 73 Non Cat
Kuma Stove Inc. Ashwood 3.5 seventy three Non Cat
Kuma Stove Inc. Scot HT-one 3.five 73 Non Cat
Kuma Stove Inc. Wooden Basic three.3 73 Non Cat
Travis Industries 42CVT Hearth .7 73 Non Cat
Unforgetable Fire Katydid one.9 73 Non Cat
Jotul North The us&nbsp 50TL two.eight seventy two Non Cat
Kuma Stove Inc. Aspen four.one 72 Non Cat
Sherwood Industries Enviro 1200, i&nbsp 3.four seventy two Non Cat&nbsp
American Strength Sys.&nbsp Minor Rascal 1.one 71 Pellet
Blaze King Industries Briarwood II/ninety&nbsp 3.5 seventy one Non Ca&nbsp
Fireplace &amp House Tech. Mt. Vernon AE 1.seven 71 Pellet
Hearth &amp Home Tech. Accentra-2 .62 71 Pellet&nbsp
Jotul North The us&nbsp C550 CB 4.five 71 Non Cat
Jotul North America&nbsp F602 CB&nbsp 3.4 71 Non Cat&nbsp
Even Temp, (St. Croix) Prescott EXP EXL .53 70 Pellet
England’s Stove Functions 15-SSW02 three.five 70 Non Cat
Quadrafire (HHT) Explorer III 2.nine 70 Non Cat
Quadrafire (HHT) Explorer II 2 sixty nine Non Cat
Wiseway Pellet Stove GW1949 one.nine sixty nine Pellet
Jotul North The usa&nbsp Castine F400 3.8 sixty eight Non Cat
US Stove Company 1269E four.2 68 Non Cat
American Power Sys. 3500, 01 &amp 02 1 sixty seven Pellet&nbsp
Fireside &amp House Tech. Advance one.eight 67 Pellet&nbsp
Pacific Energy Fireplace&nbsp FP 25 3.five sixty seven Non Cat
Even Temp (St. Croix) Hastings 1.1 66 Pellet
Even Temp (St. Croix) Ashby-P 1 sixty six Pellet&nbsp
Jotul North The united states&nbsp F370 2.6 66 Non Cat
Enjoyable Hearth, GHP&nbsp WS-2720-B 4.five sixty six Non Cat
Pacific Strength Fireplace PF16 3.one sixty five Non Cat&nbsp
US Stove Company 5660E, one.nine sixty two Pellet
Sherwood Industries EF2, Chatham,&nbsp 1.8 fifty eight Pellet

Heated Up!

Performance of Popular Pellet Stoves

This is an except of a much longer, and more technical paper by Prof. Gael Ulrich’s -“BioCombustion Institute Bulletin #3.” Gael calculated the efficiency of six popular pellet stoves, finding a wide difference.  The highest, the Italian made Piazzetta Sabrina was 76% efficient and the lowest was the Enviro M55 Insert at 51% efficient.  In between were the Ravelli RV80 (62%), Englander PDCV55 (63%), Quadrafire Mt Vernon AE (64%) and Harman Accentra 52i (71%).
He did this by using performance data produced by the Alliance for Green Heat, who tested these 6 stoves over a 30-day period.  The Alliance operated the stoves, often for 24 hours a day, testing them almost every day at various heat output settings and averaging the results. All the stoves were purchased new, without the knowledge of the manufacturers and operated with the same PFI certified pellets.  The Alliance produced an in-depth report about the findings, but we did not report the efficiency values because the instrument we used was a Testo 320, which produces a proprietary European (LHV) number, not the kind of efficiency values that are used and reported in North America. 
Gael’s full paper can be downloaded as a PDF here, which is quite technical.  We reproduced the less technical parts which are accessible to a wider audience. 

One conclusion is that many pellet stoves lack a very simple solution to increasing their efficiencies – larger heat exchangers.  Gael found that “All [the stoves], except the Enviro and Quadrafire, appear capable of adding another 5 to 10 percentage points by increasing heat exchange area to reduce the flue gas temperature.”  This solution may only add $ 100 – $ 200 to the price of a stove but would save consumers far more in fuel costs. 
One thing is clear: more expensive stoves do not necessarily provide consumers with higher efficiency. The Englander is sold by big box hardware stoves for $ 1,100, and is on par or better in efficiency than stoves that sell for $ 3,000 or $ 4,000.  This is significant because the big pellet stove manufactures do not release the actual efficiency of their stoves to consumers and consumers have virtually no way to tell which models are lower or higher efficiency.  The EPA contributed to a myth that pellet stoves have high efficiencies by giving them a default efficiency of 78%.  Emerging data shows the average pellet stove is likely around 70% efficiency, but many big name brands make pellet stoves that have efficiencies in 50s and 60s. This analysis begins to dismantle the lack of transparency in efficiency values that manufacturers have tried to maintain for many years.
Biomass Combustor Efficiency
BioCombustion Institute Bulletin #3
(Gael Ulrich: 16 March 2016)
Abstract
Gael Ulrich was a professor of
Chemical Engineering at the
University of New Hampshire

If flue gas temperature and composition are known, one can calculate the efficiency of a biomass combustor using the so-call “stack loss” technique.  This paper explains in detail why that is possible and how to do it.  Fortuitously, during the preparation of this bulletin, the Alliance for Green Heat published data from their testing of six pellet stoves this past September.[1] Test equipment used in the AGH study delivered composition, temperature, and efficiency numbers.  Investigators declined to report the efficiency numbers for various reasons, although they do mention a range of 60 to 75%. 

Using the AGH temperature and concentration data, I made independent calculations as described in detail herein.  I find one of the six stoves operating at 51% efficiency, three in the low 60s, and the remaining two operating at 71 and 76%.  I also conclude from my analysis that some of these units use “dilution as the solution to pollution.”  If we consider actual emissions in grams per hour or milligrams per MegaJoule of heat delivered instead of parts per million in flue gas, the rating is rearranged with one stove deemed second dirtiest becoming the cleanest and that ranked third cleanest becoming the dirtiest.  Factors that influence efficiency and cleanliness and how to improve these important performance properties are also discussed herein.  
Introduction
As pointed out in BCI Bulletins #1 (Units) and #2 (Emissions), biomass is intrinsically a clean fuel composed primarily of carbon, hydrogen, oxygen, and ash.  If burned properly, with ash un-entrained, flue gases from biomass can be as clean as those from natural gas and perhaps even cleaner than from oil.  The problem, of course, is that biomass is neither a liquid nor a gas like these fossil fuels.  Burning a solid cleanly and efficiently is much more difficult.  Bulletin #2 dealt with cleanliness and the standards expected.  This one focuses on efficiency and how it can be measured for a biomass burner.
Instruments and software are available to deliver efficiency ratings and other data even to an ignorant user with enough money to buy them. But to use these tools intelligently, one must know how they function and should be able to calculate efficiency separately and from scratch.  This bulletin describes how to do that. 
Fundamentals
Efficiency is a concept that everyone understands, but different people often define it differently.  Let’s solve that problem first.  For simplicity, visualize a biomass combustor as a black box with fuel and air flowing in; flue gases and ash flowing out.[2] … As defined by logic, efficiency is the ratio of useful heat released to fuel energy provided.  Fuel Energy is the Higher Heating Value,[3] a quantity that has been carefully measured over the last couple centuries by scientists for all common fuels. 
For highest efficiency,
1.  Burn with the least amount of excess air possible.
2.  Operate with the lowest feasible flue gas temperature.
3.  Use dry fuel.
Alliance for Green Heat Data
The AGH study ran for a period of 30 days.  Investigators found results that showed little drift with time.  Five of the six stoves operated with more than 200% excess air; beyond maxima considered in Figure 7.  One could derive additional curves for these high air rates just as was done for the lower percentages of excess air, but I chose to extrapolate instead, creating the dashed lines in Figure 9.
Efficiencies for the six AGH pellet stoves as read from Figure 9b are listed in Table 6. 
Table 6.  Calculated efficiencies of pellet stoves studied in the September 2015 Alliance for Green Heat test series.
Stove                O2         % X’s Air           Flue Gas            Efficiency e                             
Brand                 Conc.    (Figure 8)           Temp. (oC)         (Figure 9b)
Enviro               18.7%       800%                        150                        51 %           
Ravelli              16.8%       400%                        195                        62 %           
Englander         16.0%       315%                        222                        63 %           
Quad                  17.4%       480%                        160                        64 %           
Harman             15.0%        245%                        205                        71 %
Piazzetta            13.5%       175%                        203                        76 %
One of the six operated at 51% efficiency, three in the low 60s, and the remaining two operated at 71 and 76%.  These numbers are consistent with the range mentioned in the AGH report. 
Piazzetta achieves superiority through low excess air rate. Enviro, at the other end of the spectrum, would have an even lower efficiency if its flue gas temperature were as high as the others.  All, except Enviro and Quad, appear capable of adding another 5 to 10 percentage points by increasing heat exchange area to reduce the flue gas temperature. 
Emissions
What about Pollution?  The AGH data demonstrate an interesting application of using “dilution as a solution to pollution.”[4]  The Harman emitted flue gases containing about 820 ppm CO while the Enviro emitted 534 ppm.  But the Harman operated with about 240% excess air; the Enviro with 800%.  And, the Harman was 22% more efficient. 
At the same pellet burning rate, the Enviro produces roughly 900/340 or 2.6 times as much flue gas as the Harmon, and its useful heat delivery rate is only 82 percent as great.  Thus, in terms of mass of CO per kJ of delivered heat, a better measure of actual pollution, the Enviro is (2.6/0.82)*(534/820) = 2.1 or about twice as bad as the Harmon.  Based on the data provided, I calculated mg of CO per MJ of useful heat delivered for the six pellet stoves.  Results are listed in Table 7. 
Table 7.  Calculated CO emissions of pellet stoves studied in the September 2015 Alliance for Green Heat test series.  
                                                                                                CO emissions    
 Stove                                                                     (mg/MJ of       (ppm)
Brand       % X’s Air           Efficiency e        (ppm)    heat   normalized**
Enviro               800%                51%                  534       3000     850  (2.7)
Ravelli              400%                62%                  428       1100    365  (1.2)
Englander         320%                62%                  542       1200    387  (1.2)
Quad                  480%                64%                  318         930     318  (1.0)**
Harman              240%                71%                  821       1300     487  (1.5)
Piazzetta            170%                76%                  648         780     370  (1.2)
           
                  *Normalized to Quad as the reference.
                  **Normalized to Quad as the reference using Wikipedia formula. 
In terms of mass per unit of useful heat, the Enviro emits about four times as much CO as the Piazzetta (3000 versus 780 mg/MJ or roughly 130 versus 35 milligrams per hour).
What about non-steady-state?  My analysis assumes the appliance operates at steady state with feed rates and temperatures invariant with time.  This is valid for automatic-feed pellet stoves but not for wood stoves that are fed batch-wise.  There, the burn mode migrates from de-volatilization and combustion of light organics, gradually progressing to char or carbon burn-out.  Fortunately, stage changes are slow relative to combustion kinetics.  At any given time, the analysis described herein can be used to analyze the appliance at that instant.  To more accurately reflect the performance of a batch-fired wood burner, one must record data over a complete firing cycle and then integrate results to obtain an average.  This is further complicated by the fact that heat of combustion changes with time.  That for carbon, for instance (near burn-out), is about 30,000 kJ/kg.  Since the overall HHV for biomass is 20,000 kJ/kg, that for the volatiles must be lower than this.

What about moisture condensation? Mark Knaebe advocates improving efficiency by increasing heat exchange surface to the extent that water in the flue gas is condensed, adding its latent heat to the useful Q.  This requires dropping flue gas temperature below the dew point.  With low amounts of excess air, the dew point might be as high as 60 deg-C, but with 400% excess air, where many pellet stoves operate, the dew point is nearer 30 deg-C.  
As cleaner appliances develop, the prospect of taking advantage of this extra heat becomes more intriguing because the condensate will be purer and non-fouling.  The added heat transfer surface and increased capital cost, however, may not be practical.  
Ray Albrecht suggests that temperatures in the range of 1000oC or greater are needed to achieve good burnout of flue gases.  He stresses the importance of preserving flame temperature by insulating the combustion chamber to make sure reaction is complete before gases enter the heat exchanger.
Staging the air feed can promote gasification and partial combustion at low excess air conditions where temperatures are higher.  Preheat can almost deliver a one-to-one increase of flame temperature with increased feed air temperature.  Staging and preheat are common in newer biomass burners. 
Catalysts are another important way to promote oxidation at lower temperatures than those needed otherwise.  



[1]Alliance for Green Heat press release [Oct. 27, 2015]  http://www.forgreenheat.org/decathlon/intro.html
[2]For simplicity, assume it is burning at “steady-state” where flow rates and temperatures are constant; not changing with time.  This is true of many pellet stoves and large-scale furnaces.  Small batch-fed systems do experience cycles and are more complex to analyze, but the steady-state analysis gives a useful result even for these systems.
[3]Unfortunately, fuel energy can be expressed in multiple ways, depending on how it was measured–giving different numbers for the same fuel.  As argued in BCI Bulletin #1, HHV or the Higher Heating Value is preferred and will be the only one considered here.
[4]A phrase attributed to the 20th century comic-strip character “Pogo.” 

Heated Up!

Study of pellet stoves shows environmental advantages – and corporate exaggerations

An independent assessment of popular pellet stoves conducted by the Alliance for Green Heat  found that pellet stoves, unlike most wood stoves, can achieve low levels of emissions in real world settings that are in line with laboratory results.   
The Alliance for Green Heat ran a battery of tests on popular pellet stoves designed to approximate how they would perform in the real world.  The group found that half of the stoves operated as clean at the end of the thirty-day test than they did at the beginning and the others were only slightly dirtier.
All six stoves, from the least to the most expensive, operated well, and produced enough heat for a small to medium- sized home in most of the United States.  One of the biggest differences was that the three more expensive stoves tested (above $ 4,000) needed very little weekly cleaning and maintenance.  The less expensive stoves ($ 1,200 to $ 3,300) needed daily or at least bi-weekly cleaning of their burn pots and glass.
The study also found a lack of accepted reporting standards, leading to exaggerated claims about efficiency, BTU output and pellet hopper size on manufacturer websites and promotional literature. 
The Alliance for Green Heat tested the stoves to give consumers better tools and make better purchasing decisions.  The study is part of a yearlong Pellet Stove Design Challenge that assesses the state of existing pellet stove technologies.  The Design Challenge will culminate in a competition for the cleanest and most efficient stoves, modeled after the Department of Energy’s Solar Decathlon.
Approximately one million American homes are heated with pellet stoves, more than twice the number that have solar panels.  In Italy alone, 2 million households heat with pellets. Pellet stoves often serve as primary heat sources, enabling homes to eliminate or drastically reduce fossil heating fuel.  Last year, about 40,000 pellet stoves were sold in the U.S. and they may outsell wood stoves in the near future.
The Alliance tested the England Stove Works 25-PDCVC, the Enviro M55 insert, the Harman Accentra 52i insert, the Piazzetta Sabrina, the Quadra-Fire Mt. Vernon AE, and the Ravelli RV80.  The group assessed each stove on its cleanliness, efficiency, maintenance, heat output and visibility of glass.  The overall winner was the Quadra-Fire Mt. Vernon AE, which received top marks in three of the five categories.  The Harman Accentra received top marks in two of the five categories.

The results of this study underscore that pellet stoves tend to burn substantially cleaner than wood stoves in real world settings, but it challenges the notion that pellet stoves generally have higher efficiencies than wood stoves.  The efficiencies of the six stoves were low to medium, which is partially the result of companies not having to test and report actual efficiency numbers. 
“Our testing confirmed that pellet stoves are an effective and affordable renewable energy technology,” said John Ackerly, President of the Alliance for Green Heat. “We hoped to see higher efficiencies, but efficiencies should improve in coming years,” Ackerly added.
Click here to read the full report.
The Alliance for Green Heat promotes modern wood and pellet heat as a low-carbon, sustainable and affordable residential energy solution. The Alliance works to advance cleaner and more efficient wood heating appliances and focuses on low and middle-income families.  Founded in 2009, the Alliance is a 510(c)(3) non-profit organization based in Maryland. 

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Old-timey wood stoves need to get new-timey if we really want to make use of them

Monday, December 8, 2014
Nashua Telegraph
by David Brooks

Tom Butcher from Brookhaven Lab,
second from right, tests an automated
stove from New Zealand. Ben Myren,
left, did R&D work on it.

I don’t think very hard when I light up the old wood-burning stove in my basement. Turns out, that might be a problem.

“Combustion technology is incredibly complex. Numerous chemical engineers, combustion engineers, mechanical engineers around the world are constantly trying to understand the intricacies associated with combustion. It is absolutely not what you and I would think – just light a match … especially when you want to get clean combustion and use wood efficiently,” said Rob Rizzo, manager of the Renewable Thermal Program for the Massachusetts Department of Energy Resources.

Rizzo was among the organizers of the 2014 Stove Design Workshop held in November at Brookhaven National Laboratory in New York, the latest in a number of attempts to add some high-tech wizardry to that staple of New England life, the wood stove. (For details, see forgreenheat.blogspot.com/2014/11/rookie-wood-stove-makers-get-highest.html)

Why tinker with something as well-established as wood stoves?

Because, like me, most people don’t think too hard when using them, which makes them inefficient and polluting.

We use green wood or wet wood, and we fiddle with the damper in the wrong way, causing partial combustion and thus more pollution.

The Stove Design Workshop, like a national Stove Design Challenge in 2013 that featured two New Hampshire entries, wants to find technology that can better cope with our stupidity.

The five finalists in the design workshop used a variety of techniques to work around people, including oxygen sensors that control fuel-to-air ratio, a common emission-control technique in cars, and a New Zealand stove that has a “barometrically operated variable choke venturi tube” to control the amount of combustion air entering a stove, particularly at lower burn rates.

“The whole concept with the design challenge is to come up with solid-wood stove that eliminates the human interface. Basically hit a start button and walk away; that is the concept we’re aiming at,” he said.

This already happens with pellet stoves, of course, which is why pellets has led a wood-burning renaissance for building heat.

The drawback is that they burn pellets made of compressed sawdust rather than the wood I can snag for free off my property, especially after the Thanksgiving snowstorm knocked down so many big limbs.

The lure of free fuel means that a lot of people still burn non-pelletized wood for some or all of their heat, although it’s not clear how many.

I have never been able to find good data about people who use cordwood (a.k.a. “roundwood”) as their principal heat source, partly because it’s hard to pin down. I, for example, use it only as a minor supplement of the pellet stove in the living room and our oil-fired furnace.

Rizzo said he didn’t know any data either, but he said that wood stoves remain important, especially in western Massachusetts.

Just as important as convenience is cleanliness. Wood stoves can produce a lot of pollution, particularly fine-particle soot, that is a health hazard. This is particularly a problem around Keene, which has a lot of wood-burning stoves and a geography that traps air in certain weather conditions.

New Hampshire has used rebates to get people to turn in their old stoves for cleaner versions, although with limited success.

But those cleaner stoves aren’t all that great; they’re little more than old stoves with catalytic converters in the stovepipe. Hence the push to build a better mousetrap, so to speak.

“It’s exciting to see new ideas coming forward. We have some educated guesses but we need to do better,” Rizzo said.

“We need to collect more data, about efficiency, emissions, consumption volumes, and also source of wood, sustainability of wood source, quality of wood source. Because rural America is always going to be burning round wood.”

More info:

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EPA Declines to Release Efficiency Data on Wood Pellet Stoves

On Thursday, the EPA said it would not release efficiency data on pellet stoves, one of the most popular renewable energy technologies in American homes.  About one million homes use pellet stoves in the United States, yet none of the major stove manufacturers disclose the tested efficiency of their products. 
The EPA recently set stricter emission standards for wood and pellet stoves, but did not require the disclosure of efficiencies of stoves currently on the market.
The Alliance for Green Heat filed a Freedom of Information Act (FOIA) request with the EPA on June 3, 2015 urging the agency to release stove emission and efficiency information.  In their response letter, the EPA agreed to release detailed emission test results and other data, but denied requests for efficiency information, leaving consumers in the dark as to whether they own a 50% efficient stove or an 80% efficient stove.  Click here for a copy of the EPA letter.
The Alliance for Green Heat filed the FOIA request because stove manufacturers routinely advertise misleading and exaggerated efficiency claims.  No federal or state agency requires manufacturers to use a common method of reporting stove efficiency in their promotional materials.
Most households use pellet stoves because they can save $ 500 to more than $ 2,000 per winter by using wood pellets in place of oil, propane or electricity.  However, the potential savings fluctuate widely based on the efficiency of the stove.
The EPA said they decided not to release the efficiency data because “trade secrets and commercial or financial information are exempt from being released” to the public.  Stove manufacturers claim that their efficiency information, along with almost all other data obtained from the certification testing is “confidential business information.” 
However, the EPA left open the possibility that it might release the efficiency values if the Alliance for Green Heat wants to receive a “final confidentiality determination” that would involve giving manufacturers an opportunity to substantiate their confidentiality claims.
John Ackerly, President of the Alliance for Green Heat said that the organization intends to seek a final determination from the EPA.  “Cars, furnaces and large appliances all have to report their efficiency and energy use to the public and this is a critical part of helping consumers save money and help the environment.  Why should there be a special exemption for pellet stoves?” Ackerly asked. 
“The lack of transparency on efficiency is holding this sector, as other forms of renewable energy are quickly gaining traction,” Ackerly noted. 
The new EPA wood heater regulations will require manufacturers to disclosure the efficiency of new wood and pellet stoves tested after May 15, 2015.  However, stoves that are already on the market will not need to be tested for another 4-5 years and will not be required to disclose their efficiency until this time.  Many manufacturers obtain efficiency data during their EPA certification tests and include them in their test reports to the EPA, although they do not release them to the public.  The FOIA was an attempt to get the EPA to release this information that it already has in its files.
Pellet stoves have a very wide range of efficiencies and manufacturers may claim that if they released the actual efficiency of their stoves it cold cause substantial harm to their competitive position, which is one of the reasons the EPA withholds business information.
Stove manufacturers are also hesitant to disclose efficiency information to their consumers because if their efficiencies are under 75%, their product would not be eligible for a federal tax credit that has often been retroactively renewed and requires a 75% minimum efficiency.  Since neither Congress, nor the IRS, nor EPA requires any standardized measure of efficiency, stove manufacturers can claim that nearly 100% of stoves on the market are 75% efficient.  If the EPA were to disclose actual efficiencies, it would make it harder for manufacturers to convince consumers that all their stoves were over 75% efficient.
The number of U.S. homes with pellet stoves far outnumber those with residential solar panels by a ratio of about 2:1.  Pellet stoves typically cost $ 1,100-$ 3,500, and have a payback period of 3-7 years depending on such factors as the price of the fossil fuel being displaced, the frequency the stove is used – and its efficiency.

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Technology Design Challenge to Promote Top Performing Pellet Stoves

A yearlong project to test and assess pellet stoves is entering its first phase this summer.  This first phase focuses on the most popular pellet stoves in North America and aims to help consumers identify the most efficient stoves and how best to operate them.
The initiative is being led by the Alliance for Green Heat with a wide group of partner organizations and stove experts.  Pellet stoves are increasingly popular in North America and Europe, are far cleaner than wood stoves and have enormous potential to replace fossil heating fuel. 
2013 Design Challenge
in Wash. DC

The second phase of the project will feature an international stove technology competition to spotlight innovative and high performing pellet stoves and prototypes.  The project will culminate in an international gathering at Brookhaven National Lab in New York, where pellet stoves will be tested and the top performers will receive awards.  The focus will not just be on low emissions and high efficiency in a test lab setting, but also in the hands of consumers.  The application to submit pellet stoves and stove prototypes for the competition will be available later this summer.  The competition and workshop is scheduled for the week of April 4, 2016.

Ben Myren, Tom Butcher and others
in a lab at Brookhaven at the 2014
Wood Stove Design Challenge

This pellet stove challenge marks the third Stove Design Challenge, and the second to take place at Brookhaven. Previous challenges, including the inaugural Wood Stove Design Challengeon the National Mall in 2013, focused on promoting technological innovations that would help ensure wood stoves burn cleaner in the hands of consumers.  All the Stove Design Challenges involve stakeholder engagement in testing and assessing stoves, the opportunity to see and understand how testing works, and workshops and roundtables that bring together industry, regulators, air quality groups, non-profits and the media.

Phase one: Testing of Popular Pellet Stoves
There are approximately one million pellet stoves in North America.  Unlike the wood stove market, which has many manufacturers, most pellet stoves are made by just a handful of companies.  The Alliance will independently test some of the most popular models made in North America and Europe for emissions and efficiency.  The first round of tests will follow EPA lab testing protocols, while the second round will approximate how some consumers might use the stoves to better understand the range in efficiency when stoves are burned clean as well as dirty.  We will also test the stoves for noise level and ease of cleaning and repair. We may also test heat output and efficiency differences using high and low quality pellets.
Prior to both rounds of testing, the Alliance will explain what test methods and procedures are to be   used and seek input from industry and other stakeholders.  Testing will begin in an EPA accredited test lab and then move to Brookhaven National Lab.  The project expects to produce some data about the variability and reproducibility of emissions and efficiency in pellet stove testing. 

Planning meeting for the 2013 Design
Challenge. Pictured (left to right) Ray
Albrecht, Rod Tinnemore, Mark Knaebe,
John Ackerly, Melissa Bollman, David Agrell,
Ellen Burkhard & Tom Butcher

To get better baseline data on some popular stoves, the Alliance for Green Heat submitted a Freedom of Information Act (FOIA) request to the EPA asking for emission and efficiency data for each of the four tested burn rates.  The Alliance also asked for moisture and ash content of the pellets used in the certification tests. 

Funding for the ongoing Wood Stove Design Challenge initiative has been provided by the New York State Energy and Research Development Authority (NYSERDA), Osprey Foundation, the US Forest Service and the Alliance for Green Heat.

More information about this yearlong project will be posted on our website, blog, Facebook page and monthly newsletter.

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